Cork insulation; a complete illustrated textbook on cork insulation ...

102 CORK INSULATION
usually classed as convection, although in several respects it
differs widely from convection as just discussed. In ordinary
convection, it has been noted that the surface layer of fluid
plays an important part; but in the steam boiler the finite
layer of water next the hot boiler wall is heated and vaporized,
thus absorbing a very large amount of heat. Such steam is
instantly replaced by other water and the process is continued,
a procedure distinctly different from the usual convective
heating process and one in which the rate of heat transfer is
much higher. By drainage, on the condenser end of the
system, the film of condensed water is quickly removed,
which differs from the usual transfer by convection.
The transfer of heat by evaporation and condensation has
a definite bearing on the effect of moisture in insulating
materials and in air-space construction.
Thus, in general, the rate of heat transfer by convection
is dependent on the kind of fluid in contact, the temperature
differences, the velocity of the convecting fluid, the character
of surfaces (such as shape and roughness), and the area of
the surface.
67.
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Radiation.—Radiation is the mode of transfer of heat,
for example, from the sun to the earth, which is accomplished
even though the intervening space is entirely devoid of ordinary
matter. The transfer of heat by radiation is effected by
wave motion exactly similar in general character to the wave
motion that constitutes light, these waves being transmitted
by a medium, known as ether, that fills all space, although,
contrary to popular belief, considerable obstruction is offered
to the passage of these waves.
The molecular disturbance in a hot body produces a commotion
in the immediate adjacent ether, which spreads out in
all directions as an ether wave disturbance, and when these
waves impinge on a cool body they produce a molecular dis-
turbance in it. In a word, the heat energy of a hot body is
constantly passing into space as radiant energy in the luminif-
erous ether, and becomes heat energy again only when and as
it is absorbed by bodies upon which it falls ; and energy trans-
mitted in this way is referred to as radiant heat, although it is
transmitted as radiant energy and is transferred again into